Contribution of Hop Bitter Substances to Beer Staling Mechanisms. R. S. Williams and H. P. Wagner, Brewing Research Department, Labatt Breweries of Canada Ltd., London, Ontario N6A 4M3. J. Am. Soc. Brew. Chem. 37:0013, 1979.

Ethyl esters of isovaleric and 2-methyl butyric acids form in bottled beers and contribute to stale sensory characteristics. Previous research suggests that these acids, together with isobutyric acid, arise from oxidation of hop constituents. Therefore, a method for quantitating these acids was developed, and pilot brewery beers hopped at different levels and by various methods were studied. Although substantial quantities of the acids in fresh beers were derived from fermentation, significant amounts were contributed by oxidation of hop constituents during wort production. Hopping rate was important, but the bittering method also apparently influenced concentrations of the acids in finished beer. During warm storage, the bitterness of hopped beers decreased at rates dependent on hopping levels and the acid levels increased simultaneously. These changes correlated with the formation of ethyl isovalerate and flavor staling. During cold storage, these changes were suppressed and flavor stability was enhanced. Oxidation of hop constituents apparently contributes to flavor deterioration. Hopping rate and method also may influence physical stabilities.

Keywords: Beer stability, Iso-α-acids, Isovaleric acid, 2-Methyl butyric acid, Oxidation